In process for the recovery of zinc from zinc sulfate solution by electrolysis, it is necessary to subject electrolyte to purification in order to reduce the content of impurities that have deleterious effects in electrolysis. It is particularly important to obtain an adequate removal of monovalent ions, such as the chloride and fluoride anions and the monovalent thallium cation. The presence of chloride in zinc sulfate electrolyte causes the corrosion of lead anodes, and at high concentrations the evolution of chlorine gas could occur which would be harmful in the work environment. Fluoride is the main cause of zinc deposits sticking to the cathodes, and thallium co-deposits with zinc and also tends to raise the amount of lead deposited with zinc. Although trivalent thallium is usually removed during iron precipitation, the monovalent form remains in the purified electrolyte. The ZnSO.sub.4 purification process and the effects of various impurities are discussed in Mathewson C.H., Zinc--The Metal, Its Alloys and Compounds, 3rd Print, Reinhold Publishing Corporation, 1964, pp. 65-333.
The prior art contains a number of references on the removal of halides from ZnSO.sub.4 electrolyte. Chloride has been conventionally removed as CuCl (U.S. Pat. Nos. 903,732, 1,104 907, 1,901,925 and 4,005,174, and G.B. Pat Nos. 18,080 (1890) and 1, 491,851), or as AgCl (U.S. Pat Nos. 1,403,065 and 4,263,109), or by solvent extraction (U.S. Pat. Nos. 3,393,046 and 3,958,986, and DE Pat. No. 2,136,557). Chloride and fluoride have been removed by preleaching (U.S. Pat. No. 3,494,841) or by washing calcined concentrate, flue dust, residue, ashes and dross with sodium carbonate solution (G.B. Pat. No. 1 139 460). These methods for halide removal have a number of disadvantages, the main one being that the removal is usually incomplete and that methods for removal of chloride and fluoride are usually complemented by further removal of chloride as CuCl or AgCl. The removal of fluoride by washing solids with soda solution is expensive and will still result in fluoride ions being present in the purified ZnSO.sub.4 electrolyte. It is generally recognized in the industry that no methods exist that can effectively and cheaply remove fluoride ions from ZnSO.sub.4 electrolyte.
The removal of ions from solutions could be carried out by methods that may include electrodialysis. Removal of anions and cations from solutions by electrodialysis is well documented (e.g., see U.S. Pat. Nos. 3,673,067; 3,926,759, 4,207,157 and 4,325,792; JA Kokai No. 79 14323 (Chem. Abstracts 90, 159167q, 1979); SU Pat. No. 1 042 770 (Chem. Ab. 99, 197263m, 1983); and Chem. Ab. 88, 158970t, 1978). These references, however, do not disclose the use of electrodialysis in the removal of thallium from solutions, or the use of electrodialysis in the purification of ZnSO.sub.4 electrolyte for the electrowinning of zinc, or the removal of chloride, fluoride and thallium from ZnSO.sub.4 electrolyte by electrodialysis.